Method of minimizing &#34;patchiness&#34; in etched chromium electrodeposits



Patented Feb. 14, 1950 METHOD OF MINIMIZING PATCHINESS IN ETCHED CHROMIUM ELECTRODEPOSITS George Dubpernell, Wa to United Chromium,

tertown, Conn., assignor Incorporated, New York,

N. Y., a corporation of Delaware No Drawing. ApplicationMay 20, 1947,

' Serial No. 749,338 I 1 Claim. (Cl. 20435) This invention relates to methods of minimiz ing .patchiness in etched chromium. electrodeposits, and provides improvements therein.

Methods are known for producing so-called porous chromium by etching thereof; vide Van der Horst U. 5. Patent 2,314,604, and the Webersinn'and Hyner U. S. Patent2,430,750. Within the knowledge of the art are the conditions for electrodepositing chromium which predispose the electrodeposit to the formation of fissure networks on etching, and various methods and means of etching the predisposed chromium electrodeposit to develop the-fissure network. The chromium electrodeposit, after etching, is mechanically finished (usually by grinding, polishing or honing). The finished chromium has utility for resisting frictional wear, and is extensively used on the bores of cylinders of internal combustion engines, piston rings, thread guides, and many other uses where resistance to frictional wear is important. The so-called etched porous chromium is ,usually produced of two types, one known as the .pitted type, and the other as the channel or mud-crack type.

In producing etched porous chromium a condition known as patchinessis encountered and until now it has not been known how to overcome such condition. Patchiness is the result of the uneven action of the etching. This uneven action consists of either or both a different rate of etching at different areas, or a varying time-lag in the commencement of the etching action at different areas. The result is patches of surface area etched to slightly different levels or degrees. After finishing (honing for example) the surface presents a patchy appearance which is objectionable, and which in severe cases impairsthe functioning of the chromium as a wearing surface.

I have discovered a simple and efiective method of minimizing patchiness.

I have discovered that by reducing, at or during the end-part of the period in which the chromium is being electrodeposited, the cathode current density to a low value with respect to the current density at which the chromium has been electrodeposited, patchiness is eliminated or minimized when the chromium s electrodeposited is etched.

The amount (or density) of the current in the aforesaid terminal part of the electrodeposition need not be sufficient to actually deposit chromium, but can during the whole or a part of said end-part of the electrodeposition be below an amount at which chromium is electrodeposited.

The desired effect is obtained in the same ba h in which the electrodeposition of the chromium is done and without interruption of the current. The predisposed chromium. electrodeposit may then be transferred to the tank in which the etching is done without intermediate treatment other than the usual rinsing with water. The chromium electrodeposit to which my invention is applied may be any one Whichis predisposed to-the'formation of either pitted type or channel type porosity, electrodeposited-from a chromic acid catalyst acidradical plating bath by any known method for producing such predisposed chromium. In such known methods, the ratio of chromic acid-to sulphate acid radical ranges from 25 to 1 to 150 to 1; the temperature ranges from 71 C. (160 F.)'to 25 C. (77 'F.); and the current densities are of the ranges usually employed in commercial industrial or so-called hard plating, as described in my article on Chromium Plating in Modern Electro'plating published by The Electrochemical society, in 1942, Tables II and III, pages 122 and 123, a preferred range of current densities-being from A to 16 amperes per square inch. 7 e

In the practical art of electrodepositing chm-- mium predisposed to the formation of so-called porous chromium, CIOs to S04 ratios from 25 to 1 to 125 to 1, temperatures C. to 65 C. and current densities from 2 to 5 amperes per square inch, are those usually employed.

For so-called pitted type porosity a specific mode of procedure for plating chromium is as follows:

CrOs, 250 g./l.

Ratio, 40 to 1 Temperature, C.

Current density, 3 amps/sq. in. Time, 55 min/thousandth inch For so-called channel type porosity a specific mode of procedure for plating the chromium is as follows:

CrO3, 250 g./1.

Ratio, 115 to 1 Temperature, C.

Current density, 3.5 amps/sq. Time, 40 min/thousandth inch the plating conditions, principally current density and temperature, employed for obtaining the desired thickness of electrodeposit, all of which is well within the knowledge of. the art.

According to the present invention, an electrodeposit of chromium,- plated :under conditions which predispose it*to. the formation of either pitted type or channel type porosity, while in the tank in which the plating is done, with out interruption of the currentfloy nhas the cur:

rent flowing thereto reduced,tozsuchianamountv that the cathodic current density corresponds to a plating current efliciency: of;6%:to. 0 and the current flow continued at; the reduced current" density for about one to ten minutes, after which the chromium electrodeposit is removed fromthc plating solution, and etched and otherwise treated, as may be required to produce the so-called etched "porous chromium? The reduced current density at the aforesaid'rend-period in the plating tank? need :'not' be; sufiicient to actually electrodeposit chromium: Fiveminutesyajs a general rule, is a goodlengthofrtime for continuing the currentflow. at. the reduced current density, ,for practical use. .7 t

. Example 4 .l a

Chromium had; been electrodeposited ,for; 9 hours on steelfrom a;250-g./l. chromic acid plating bath having, a CrOr to S04 ratioof ,115 to 1, temperature 50 0. (122 F.) and a'mean normal cathode current density of ,3. amperes persquare inch.

cathode current density was :reduced to 0.4 ampere per square inch (current efiiciency 3.l%) andtthewcurrent flow continued at said reduced currentdensity for'5 minutes. Current densities of 0.1 to -1 ampere per square inch, have also been used." The chromium :plated article was then removed from the platingbath and rinsed.

Formation of fissure networkszwas; carried out on these, chromium electrodeposits following known procedures. Specifically, some were made cathode in-a-hot solution (70C.) of oxalic acid *(100 g./l.) atone-quarter ampere .per square inch, for 30 minutes; others were immersed in a-cold Without interrupting the flow of current,'the

1:1 hydrochloric acid solution (12 Beaum) for 10 minutes; and others were treated anodically in a chromic acid solution for 25 ampere-minutes per square inch.

Chromium electrodeposits, after treatment in accordance with' -the foregoinggattreduced current density in the end-part of the plating period, on and after etching and honing, have all shown an absence of patchiness. Commonly, etched porous chromiumip foduced under the same conditionsmbutrgwithout the aforesaid end-period treatment exhibited patchiness.

What :-isclaimedsis "A method-*of-minimizing patchiness in porous chromium consisting essentially of electroplating chromiumpredisposed to the formation of porous chromium in a plating bath having ClO3/SO4 ratios from about 25 to 1 to 150 to 1, at tem-- peratures of about 77 F. to 160 F., and current densities from about Mgto 16 amperes per square inchiappropriatetorxthe temperature Oil the bath, and'aetchingnthesaid chromium electrodeposits to form .porous' chromium,- characterizedby con tinuing thepassage: of current in thesplatingbath, in-the end part of: theplating: period,"-to-the chromium alreadydeposited, at; azreduced :cathodic. current density corresponding" to a plating clfiirrentefiiciency of 6% t 0%"for-lkto'10 minu es: irg -11: .:y: w).

GEORGE DUBPERNELL.-

W HE BEN E L-Q I i The following references are of recordin: the file of this patent;

umtqjsi iiastime Number Name Date- 2,'182;244- Beall Dec; 5; 1939 40 FoRrqIG PATENTS a a N umberr 1 Country Date 7 1 335,161" Great Britain Sept. 15', 1930 HER E ERE CE 1 Thompson: heoretical and. '::ADp1 d t chemistry, 1925, pages 124 and"125.; 

